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I read (second comment to this question, which doesn'r offer no solace ar all)

If there are two people who are exactly the same then either they are the same person (via the law of identity) or they are different people.

This sounds like non-sense to me. How can two co-existing people who are exactly the same be the same person? How can a law of identity prevent this? Doesn't the fact that two persons who have a separate existence (in space or in time or in spacetime) already makes them two non-identical persons? What is this "law of identity"?

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  • The Law of Identity says: For all X ( X = X ). It is one of three Laws of Thought.
    – nwr
    Jul 11, 2021 at 22:46
  • Correct; if "they" are two spatio-temporal distinct entities, they are two and not one.
    – Mauro ALLEGRANZA
    Jul 12, 2021 at 5:56
  • This looks like a garbled version of Leibniz's law of identity of indiscernibles mixed with the "law of identity" in old logic. Identity of indiscernibles states, roughly, that if two things are distinct then there is a property distinguishing them (it can be spatiotemporal location for otherwise identical copies). Contrapositively, if two things have all properties identical then they are identical. If the class of properties considered is too small this can be violated.
    – Conifold
    Jul 13, 2021 at 0:51

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It is notoriously difficult to state the law of identity in a non-circular way. It is sometimes expressed as every individual is identical with itself and not identical with any other individual. But this in turn requires us to understand what 'self' and 'other' mean. Also, it is an open question as to what are the individuals to which the law applies.

It is often understood in the context of the principle of the indiscernibility of identicals, i.e. that if x and y are identical then they share all properties in common. This also is potentially circular, because being self-identical is arguably itself an attribute that is required to characterise identity adequately.

Some philosophers (e.g. Peter Geach) have argued that there is no such thing as an absolute concept of identity, and we can only speak of relative identity. Others (e.g. Wittgenstein) have argued that to speak of identity as an attribute is misleading.

There is a lot more information in the Stanford Encyclopedia article on Identity.

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  • "every individual is identical with itself and not identical with any other individual" No, the law of identity doesn't say anything about the relation between two things, let alone that they cannot be identical.
    – Speakpigeon
    Jul 13, 2021 at 10:13

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